The nation's campaign to get more teenagers to delay sex and to use condoms is faltering, threatening to undermine the highly successful effort to reduce teen pregnancy and protect young people from sexually transmitted diseases, federal officials reported yesterday.

New data from a large government survey show that by every measure, a decade-long decline in sexual activity among high school students leveled off between 2001 and 2007, and that the rise in condom use by teens flattened out in 2003.

Moreover, the survey found disturbing hints that teen sexual activity may have begun creeping up and that condom use among high school students might be edging downward, though those trend lines have not yet reached a point where statisticians can be sure, officials said.

"The bottom line is: In all these areas, we don't seem to be making the progress we were making before," said Howell Wechsler, acting director of the division of adolescent and school health at the federal Centers for Disease Control and Prevention, which conducts the survey. "It's very troubling."

Coming on the heels of reports that one in four teenage girls has a sexually transmitted disease and that the teen birth rate has increased for the first time in 15 years, the data are triggering alarm across the ideological spectrum.

"We have a number of signs that are all going exactly in the wrong direction," said Sarah S. Brown, chief executive of the National Campaign to Prevent Teen and Unplanned Pregnancy. "All of us in this field are on red alert."

The new report did not examine the reason for the trends, but experts said there could be many causes, including rising complacency about HIV and AIDS, changing attitudes about sex and pregnancy, shifts in ethnic diversity, and the possibility that there will always be teens who cannot be persuaded to wait.

"The truth is that, as a field, we really don't know what the answer is," Brown said. "There are lots of theories: the economy, classroom education, the messages kids are getting in the digital world where they spend their time. They probably all play a role."

The new figures renewed the heated debate about sex-education classes that focus on abstinence until marriage, which began receiving federal funding during the period covered by the latest survey and have come under increasing criticism that they are ineffective.

"Since we've started pushing abstinence, we have seen no change in the numbers on sexual activity," said John Santelli, chairman of the department of population and family health at Columbia University. "The other piece of it is: Abstinence education spends a good amount of time bashing condoms. So it's not surprising, if that's the message young people are getting, that we're seeing condom use start to decrease."

"Contraceptive sex education does not provide practical skills for maintaining or regaining abstinence but typically gives teens a green light to activity that puts them at great risk for acquiring STDs or which serve as gateway-to-intercourse activities," said Valerie Huber, executive director of the National Abstinence Education Association.

Others blamed the onslaught of movies, books, advertising and cultural messages that they say glamorize sex.

"The No. 1 movie that all teenage girls want to see right now is 'Sex and the City,' " said Charmaine Yoest, a spokesman for the Family Research Council. "Our culture continues to tell them the way to be cool is to dress provocatively and to consider nonmarital sexual activity to be normative."

The proportion of teenagers reporting having sexual intercourse rose steadily through the 1970s and 1980s, fueling a sharp rise in teen pregnancy. The trend reversed around 1991 because of AIDS, changing mores about sex and other factors. At the same time, more sexually active teens started using condoms and other contraceptive measures. Together, the trends have pushed the U.S. teen pregnancy rate to historic lows.

The first sign that trend might be reversing came last summer, when the CDC conducted an analysis for The Washington Post of data collected in 2005 by the Youth Risk Behavior Survey, a nationally representative survey the CDC conducts every two years to track risky behaviors. While the rates still remained far below the all-time highs, the analysis showed that the proportion of teens who said they had ever had sex or had had sex in the last three months had leveled off, beginning in 2001. Researchers, however, were waiting for the next round of data to see whether a real trend was represented.

The new data come from the 2007 survey, which involved 14,103 students in grades nine through 12 at 157 high schools nationwide. The survey found a slight increase between 2005 and 2007 of the proportion who reported they had ever had sex, had begun having sex before age 13, had engaged in sex within the last three months and had had sex with at least four partners.

None of the increases was sufficient to convince statisticians that there is an upward trend. But when the agency analyzed the numbers for The Post, statisticians found that every measure of sexual activity passed the statistical test for having leveled off between 2001 and 2007, and the condom use numbers passed the test for leveling off beginning in 2003.

Because the survey involved only teens who are still in school, it might underestimate the level of sexual activity and overestimate condom use, several experts said.

"What's really important here is we're really running out of steam," Wechsler said. "There's no reason for panic, but there is reason for concern."